Break the chocolate up into pieces and put it and the milk in a pan on a low heat. When the chocolate has melted, take the pan off the heat and add the butter in pieces.
Beat the egg yolks in a bowl with the sugar. Then add the chocolate mixture and stir well.
Beat the egg whites until they are stiff. Add a pinch of salt while you are beating them.
Once the chocolate mixture is cool, stir in the beaten egg whites. Stir gently, trying not to break the egg whites foam. Act as if you are stroking the mixture, stir gently and slowly until the mixture starts to have an even colour and texture.
Spoon the mixture into individual containers (small bowls, champagne glasses...) and place in the fridge. The mousse should stay for at least 2 hours in the fridge cooling before it is served.
Tricks and secrets:
If you can get hold of it, for the best results use Chocolate Valor chocolate.
If you want to get a smooth, light mousse you must be patient when you are mixing the egg whites with the rest of the mixture. If you stir slowly and gently with great patience, you will be abel to taste the results.
When you add the milk to the chocolate at the beginning, you can add a couple of drops of cognac or cointreau.
To serve, decorate the mousse with chocolate hundreds and thousands, or a sprig of mint, or fresh cream, or biscuit.